Communication systems typically comprise transceivers, which in turn include transmitters and receivers. Transmitters may transmit signals at various power levels depending on the distance that the transmitted signal has to travel to reach one or more target receivers. Satellite communications, for example, may require transmission power levels of the order of tens to hundreds of watts, whereas transmitters for short distance applications may operate at a few watts or even milliwatts. High power transmitters may use high power amplifiers (HPAs) to increase the power levels of the transmitted signals to desirable levels for long distance transmission. Design of HPAs, especially for linear and low voltage operations is a challenging task.
Many transmitter devices use one or more modulation schemes in order to modulate a high frequency carrier waveform at a frequency of ωc by one or more original signals (i.e., baseband signals) to generate a passband signal. The passband signal may have a spectrum that is nonzero in a band around ωc. Analog modulation may be implemented to modulate the amplitude (i.e., amplitude modulation (AM)), phase (i.e., phase modulation (PM)), or frequency (i.e., frequency modulation (FM)) of the carrier wave with the baseband signal. In digital modulation, the carrier frequency is modulated by a digital baseband signal. Digital modulation may offer several advantages over analog modulation and is more common in today's communication systems such as wireless systems. Digital modulations may include amplitude shift keying (ASK), phase shift keying (PSK), and frequency shift keying (FSK). Various modulation schemes may be characterized by different values of parameters, such as signal quality, spectral efficiency, and power efficiency. Many digital communication systems may benefit from quadrature modulation, in which a binary data stream may be subdivided into pairs of bits that can be impressed upon orthogonal carrier waveforms with the same frequency, such as sin ωct and cos ωct waveforms. Modulation schemes such as quadrature PSK (QPSK), offset QPSK (OQSK), QASK (also known as QAM) are commonly used in many communication systems.
Improvement in one or more of the parameters of interest, such as signal quality, spectral efficiency, or power efficiency in transmitters using the more popular QPSK modulation is desired.